Density functional theory calculations have been employed to study the effects of alloy on energetics and preferential adsorption sites of atomic (H, C, N, O, S), molecular (N(2), NO, CO), and radical (CH(3), OH) adsorption on RhMn(111) alloy surface, and underlying electronic and structural reasons have been mapped out. We find that though Mn is energetically favorable to stay in the subsurface region, the RhMn surface alloy may be developed via the segregation induced by strong interaction between oxygen-containing species and Mn. Independent of adsorbates (not including O and OH), the interactions between these species and Rh atoms are preferential, and enhanced in general due to the ligand effects induced by Mn nearby. In contrast, oxygen-containing species (atomic oxygen and hydroxyl) prefer to coordinate with Mn atom due to the significant hybridization between oxygen and Mn, a manifestation of the ensemble effects. The order of the binding energies on RhMn alloy surface from the least to the most strongly bound is N(2)< CH(3)< CO < NO < H < OH < O < N < S < C, which is also found on Rh(111) surface, due to the distinct reactivity of these species overwhelming the ligand/ensemble effects present in surface alloy. The implication of the modification of the adsorption energy, site preferences, and their relative stability on RhMn alloy surface, on the syngas (CO+H(2)) selective conversion, are discussed.